Fusidic acid (FA) is a tetracyclic triterpenoid or fusidane (steroidal) antibiotic derived from the fungus Fusidium coccineum that inhibits bacterial protein synthesis. FA is effective against gram-positive bacteria such as Staphylococcus species and Corynebacterium species (L. Verbist, J. Antimicro. Chemo. 25, Suppl. B, 1-5 (1990); A. Bryskier, Fusidic Acid, Chapter 23, in Antimicrobial Agents: Antibacterials and Antifungals (Andre Bryskier, Ed., ASM Press, Washington, USA, 2005)). FA also has moderate activity against Group A beta-hemolytic streptococci, or Streptococcus pyogenes (L. Verbist, J. Antimicro. Chemo. 25, Suppl. B, 1-5 (1990); A. Bryskier, Fusidic Acid, Chapter 23, in Antimicrobial Agents: Antibacterials and Antifungals (Andre Bryskier, Ed., ASM Press, Washington, USA, 2005); Skov et al., Diag. Micro. Infect. Dis. 40:111-116 (2001)).
FA was developed for clinical use in the 1960s and it is approved for human use outside of the United States, such as in the UK, Canada, Europe, Israel, Australia and New Zealand. It is typically prescribed at doses of 500 mg TID for treating skin and skin structure infections caused by Staphylococcus aureus (A. Bryskier, Fusidic Acid, Chapter 23, in Antimicrobial Agents: Antibacterials and Antifungals (Andre Bryskier, Ed., ASM Press, Washington, USA, 2005); Collignon et al., Int'l J. Antimicrobial Agents 12:S45-S58 (1999); D. Spelman, Int'l J. Antimicrobial Agents 12:S59-S66 (1999)), although some physicians have routinely prescribed the compound at 500 mg BID for treating skin and skin structure infections due to the long half-life of the compound (Fusidic Acid, in Principles and Practice of Infectious Diseases, 6th ed. (Mandell et al. eds., Elsevier, 2006)).
Treatment using FA has been well studied and it is generally regarded as safe when administered to humans, as evidenced by the fact that the drug has been in continuous use for more than 40 years. There are, however, several characteristics of FA that have prevented use of the drug against a wider spectrum of bacteria and in the treatment in additional types of infection. For example, approved dosing regimens have been shown to select for bacterial resistance, such as in S. aureus. Approved dosing regimens provide low multiples of the MIC and as a result, S. aureus resistant mutants can be selected after the first day of dosing. Once resistance has developed, FA is not effective against the resistant strains. Resistance is reported to occur if FA is used as a single drug as the resistance frequency at 4 and 8 times the MIC is in the range of 10−6 or 10−8 (Evans et al., J. Clin. Path. 19:555-560 (1966); Hansson et al., J. Mol. Biol. 348:939-949 (2005), Jensen et al., Acta Pathol Microbiol Scand. 60:271-284 (1964); Besier et al., Antimicrob. Agents Chemo., 49(4):1426-1431 (2005); Gemmell et al., J. Antimicrobial Chemo. 57:589-608 (2006)).
The dosage of the drug cannot be simply increased as a means of avoiding development of resistance. It is difficult to achieve high concentrations of FA in the blood due to the substantial protein binding of the drug (approximately 95-97%) (K. Christiansen, International Journal of Antimicrobial Agents 12:S3-S9 (1999); Coutant et al., Diagn Microbiol Infect Dis 25:9-13 (1996); D. Reeves, J. Antimicrob. Chemo. 20:467-476 (1987); J. Turnidge, Int'l J. Antimicrobial Agents 12:S23-S34 (1999); Rieutord et al., Int'l J. Pharmaceutics 119:57-64 (1995)). Moreover, high dosages of FA are not well-tolerated by patients receiving the drug. High doses of FA (e.g., 1 gram TID) are required if the drug is to be used in the treatment of bone and joint infections, less susceptible bacteria and other serious infections. However, treatment regimens using high doses of the drug induce nausea and vomiting and are rejected by patients (Fusidic Acid, in Principles and Practice of Infectious Diseases, 6th ed. (Mandell et al. eds., Elsevier, 2006); K. Christiansen, International Journal of Antimicrobial Agents 12:S3-S9 (1999); Nordin et al., Eur. J. Clin. Res. 5:97-106 (1994)).
In view of the tremendous costs associated with the de novo development of new anti-bacterials, expanding the indications for drugs that have already been demonstrated to be safe and effective is strongly needed. Overcoming the limitations on the uses of FA would broaden the population of bacterial infections against which it could be used and thus meet this need.